2-methylbutanal (2-methylbutyraldehyde) is a chemical intermediate that is useful in the synthesis of isoprene (2-methyl-1,3-butadiene). Isoprene is in turn useful as a monomer in the synthesis of synthetic rubber.
Hydroformylation reactions, commercially known as the oxo process, were discovered by O. Roelin in Germany and patented in 1943 (see U.S. Pat. No. 2,327,066). Hydroformylation is the addition of hydrogen and carbon monoxide to an alkene to form an aldehyde which can be further reduced to an alcohol. Hydroformylation reactions are carried out in the liquid phase in the presence of transition-metal catalysts. For example, olefins can be hydroformylated into aldehyde by treatment with carbon monoxide and hydrogen in the presence of a transition metal catalyst (cobalt carbonyl), as is shown below. ##STR1##
Alcohol can be obtained by allowing this reaction to continue after all the carbon monoxide (CO) is used up, see the reaction shown below. ##STR2##
In light of this hydroformylation chemistry the synthesis of pure 2-methylbutanal would appear to be quite simple. However, in practice the synthesis of relatively pure 2-methylbutanal from 2-butene cannot be achieved by employing standard oxo processes. The problem encountered is that attempts to hydroformylate 2-butene into 2-methylbutanal result in the formation of n-pentanal as well as 2-methylbutanal. Thus, mixtures of 2-methylbutanal and n-pentanal are produced in lieu of pure 2-methylbutanal. The hydroformylation of 2-butene can result in the formation of mixtures containing over 25 weight percent n-pentanal unless the techniques of this invention are employed.
Many transition-metal ions and complexes, especially of Group VIII metals promote double bond migration in alkenes. Thus, a possible explanation for the formation of n-pentanal is that 2-butene is isomerized into 1-butene in the presence of transition-metal hydroformylation catalysts with the 1-butene then being hydroformylated into n-pentanal and 2-methylbutanal. In other words, it is theorized that 2-butene is isomerized into 1-butene under the conditions employed in an oxo process with the 1-butene then being hydroformylated into a mixture of normal-pentanal and 2-methybutanal. The reaction scheme shown below as FIG. 1 illustrates three possible reaction paths that can be taken in the hydroformylation of 2butene. ##STR3##
As can clearly be seen, if 2butene is hydroformylated, as is shown as Path A, then the only possible reaction product is 2-methylbutanal. In the presence of the transition-metal catalysts used in hydroformylation reactions 2-butene can isomerize into 1-butene as is shown in the reaction scheme, thus making Path B and Path C possible. If Markownikoff addition is followed, as illustrated in Path B, then a branched-chain product will be produced (2-methylbutanal). On the other hand, if anti-Markownikoff addition is followed as illustrated in Path C, then a straight-chain product (normal-pentanal) will be formed. The hydroformylation of 1-butene normally produces both the straight- and branched-chain products. Normally in a hydroformylation reaction of this type one would expect to observe a ratio of straight-chain products to branched-chain products of about 3:1. In other words, anti-Markownikoff addition is favored over Markownikoff addition by a 3:1 margin. Thus, Path C is preferred to Path B by about 3:1. Therefore, the hydroformylation of 1-butene will normally result in the formation of about 75 percent n-pentanal and 25 percent 2-methylbutanal. The literature indicates that this ratio of straight-chain (normal) to branched chain products can be shifted to favor straight-chain product by an even greater margin if the hydroformylation is done in the presence of high concentrations of triphenylphosphine, see C. U. Pittman & A. Hirao, Journal of Organic Chemistry, Vol. 43, No. 4, p. 640 (1978).
In order for the oxo process to be used on a commercial basis for synthesizing 2-methylbutanal from 2-butene it is most important to inhibit reaction Path C. This is because it is highly undesirable to produce n-pentanal in the hydroformylation of 2-butene which is intended to produce 2-methylbutanal. By practicing the process of this invention the amount of n-pentanal produced in the hydroformylation of 2butene can be greatly reduced. Thus, this invention allows for the hydroformylation of 2-butene to provide 2-methylbutanal that contains a relatively small amount of n-pentanal.